This invention relates to a high efficiency medical surgical transducer with an ergonomically enhanced shape. More particularly, this invention relates to a device that will transform electrical signals to mechanical vibrations to allow for ablation of tumors and other unwanted body tissues while allowing line of sight visualization of the operative sight by the surgeon.
Over the past 30 years, several ultrasonic tools have been invented which can be used to ablate or cut tissue in surgery. Such devices are disclosed by Wuchinich et al. in U.S. Pat. No. 4,223,676 and Idemoto et al in U.S. Pat. No. 5,188,102.
In practice, these surgical devices include a blunt tip hollow probe that vibrates at frequencies between 20 kc and 100 kc, with amplitudes up to 300 microns or more. Such devices ablate tissue by either producing cavitation bubbles which implode and disrupt cells, tissue compression and relaxation stresses (sometimes called the jackhammer effect) or by other forces such as micro streaming of bubbles in the tissue matrix. The effect is that the tissue becomes liquefied and separated. It then becomes emulsified with the irrigant solution. The resulting emulsion is then aspirated from the site. Bulk excision of tissue is possible by applying the energy around and under unwanted tumors to separate it from the surrounding structure. The surgeon can then lift the tissue out using common tools such as forceps.
The probe or tube is excited by a transducer of either the piezoelectric or magnetostrictive type that transforms an alternating electrical signal within the frequencies indicated into a longitudinal or transverse vibration. When the probe is attached to the transducer, the two become a single element with series and parallel resonances. The designer will try to tailor the mechanical and electrical characteristics of these elements to provide the proper frequency of operation. Most of the time, the elements will have a long axis that is straight, as shown in FIG. 1. This is done for simplicity and economic considerations. In almost all applications, whether medical or industrial, such an embodiment is practical and useful. However, in applications such as open field brain surgery, such an embodiment is impractical since the doctor is using a microscope while operating, to enlarge the view of the delicate structures of the brain. Here, the length of the transducer/horn combination may be disadvantageous, since the proximal end of the transducer will contact the microscope head and interfere with the ability of the surgeon to manipulate the tool for maximum efficacy. As important, the transducer housing major diameter interferes with the surgeon's field of view of the operative site.
In the past, several inventors have attempted to solve the problem by kinking or bending the transducer or probe element to provide an angled handpiece. With this method, the surgeon handles the distal end of the combination normally while the transducer lies along his or her hand, away from the microscope head and thereby increasing the ability to visualize the operative field. FIG. 2 shows an ultrasonic transducer and probe assembly with a kink or a bend in the front driver of the transducer assembly.
Several factors have limited the benefit of a bent transducer or probe. One is the fact that the bend introduces a vector force that manifests itself as a transverse or bending wave motion. This motion reduces the efficiency of the tip action and increases the energy loss in the transducer itself. As a result, the transducer temperature rises, causing the surface to become too hot to touch. Also, the transverse vibrations lead to large stresses in the vibratory elements which at higher amplitudes cause metal fatigue and probe fracture. The transverse vector increases in direct proportion to the angle of curvature. Because of these design problems, the designer will both limit the bend angle as well as reduce the maximum tip amplitude at which the device will be allowed to vibrate. As an example, one commercially available device gives a maximum amplitude for a straight transducer probe combination as 355 microns while offering a transducer with a 10° angle for the same purpose at only 183 microns, or almost 50% less. Both remedies reduce the efficacy of the operative procedure in that the harder, denser tumors require higher amplitudes and more power to ablate and remove. In addition, the small bend angle still allows the transducer proximal end to contact the microscopes in practice.
The diameter of the transducer body is also a factor in the ergonomics of the device. The larger the unit, the heavier and more difficult it is to manipulate. When poled, most surgeons requested a device that is the size of a large writing pen. Since the electrical power required to ablate tissue and overcome the electromechanical losses in the handpiece is up to 70 watts, making a thinner handpiece that does not get hot during use is problematic due to the fact the crystal mass in a piezoelectric handpiece is reduced. The power density will then rise, increasing power loss and waste heat generation. Similar problems exist for magnetostrictive devices, although these can generally be thinner for given wattage output. However, since magnetostrictive devices cannot easily accommodate a central aspiration port (one that is concentric with the long axis) tissue blockage can occur when aspirating tissue. This is a major detriment.
Other factors, which are desirable in a practical embodiment, would be a fluid passageway with no joints within the transducer case to prevent liquid leakage into the interior of the transducer that would cause failure of the electrical components. In addition, the case of the unit should be isolated from the vibrations of the probe and transducer itself. If the case vibrated in sympathy with the transducer, the surgeon would feel the vibrations in his or her hand. This leads to less tactile feedback during the operation, fatigue and could in fact lead to damage of the hand upon long exposure.